U.S. Pat. No. 3,842,194 of Clemens discloses a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of about 2.7 micrometers and groove depths of about 0.5 micrometer may be used. During playback, a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the prerecorded information.
Keizer, U.S. Pat. No. 4,162,510, which is incorporated herein by reference, discloses a playback stylus which may be used with the Clemens system. The keel tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion and shoulders interconnecting the body with the constricted terminal portion.
The constricted terminal portion is defined by a prow, a substantially flat rear surface remote from said prow, side surfaces extending from the side edges of the rear surface, a bottom surface, which preferentially conforms to the groove shape, extending from the bottom edge of the rear surface and a plurality of additional surfaces extending from the prow and intersecting the bottom and the side surfaces. The maximum separation between the substantially parallel side surfaces is less than the given groove width.
A method for manufacturing the keel tipped stylus is disclosed by Keizer in U.S. Pat. No. 4,104,832 also incorporated herein by reference. A tapered dielectric support element, made from a hard dielectric material such as diamond or sapphire, is contacted with an abrasive lapping disc having a deep, trapezoidal coarse pitched groove. The lands on the lapping disc lap the shoulders of the keel tipped stylus and the walls of the abrasive groove form the substantially parallel side surfaces of the constricted terminal portion.
Abrasive discs can be generated by a method described by Ryan, "Material and Process Development for Video Disc Replication," RCA Review, Vol. 39, pages 87-115 (1978). A master recording is preferably made in photoresist by optical recording techniques, although other recording methods such as electron beam or electromechanical may also be employed. A metal matrix master, which is a negative replica of the surface of the photoresist master recording, is prepared from the master recording. A metal matrix mold, which is a positive replica of the master recording, is prepared from the matrix master. A metal stamper is then prepared from the matrix mold. The stamper is a negative of the master recording and is used to reproduce the appropriate groove pattern in the surface of plastic discs, which are then coated with an abrasive.
The copending application of Carroll et al. Ser. No. 118,088, filed Feb. 4, 1980, entitled "Method for Preparing Stylus Lapping Discs" disclosed a method of replicating a spiral groove pattern wherein a metal article generated during the replication process has the junctions between its surfaces filled in so that concave junctions result. Playback styli having improved lifetimes result when lapping discs fabricated by the Carroll et al. method are used. Also, metal parts generated during the replication process separate more easily.
Another problem has been encountered during the replication process. It is sometimes difficult to obtain a metal part which reproduces the pattern recorded in the photo-resist master.